Determining the ideal measurement site and respiratory condition for liver transient elastography: toward clinical practice standardization

Objectives Liver transient elastography (TE) has been endorsed by the WHO as the first-line diagnostic tool for liver diseases. Although unreliable and invalid results caused by intercostal space (ICS)-associated factors (including excessive subcutaneous fat and a narrow ICS relative to the transducer size) and operator inexperience are not uncommon, no standard guidelines for ideal probe placement are currently available. Herein, we conducted a prospective observational study to identify an ideal measurement site and respiratory condition for TE by characterizing anatomical and biomechanical properties of the ICSs using ultrasound B-mode and elasticity imaging. Methods Intercostal ultrasound was performed pointwise at four specific sites in 59 patients to simultaneously measure the width, stiffness, and skin‒liver capsule distance (SCD) of the ICSs over the liver, under end-inspiratory and end-expiratory conditions. Intersections between the 8th ICS and anterior axillary line, the 7th ICS and anterior axillary line, the 8th ICS and mid-axillary line, and the 7th ICS and mid-axillary line were defined as Sites 1 to 4, respectively. Results Results indicated that Sites 2 and 3 presented greater intercostal width; Sites 3 and 4 displayed lower intercostal stiffness; Sites 2 and 3 exhibited a shorter SCD. The ICSs were significantly wider and stiffer at end-inspiration. Additionally, the liver was more easily visualized at Sites 1 and 3. Conclusion We recommend Site 3 for TE probe placement owing to its greater width, lower stiffness, and smaller abdominal wall thickness. Performing TE at end-inspiration is preferred to minimize transducer-rib interferences. This study paves the way toward a standardized TE examination procedure. Critical relevance statement A standardized measurement protocol for WHO-recommended liver TE was first established to improve the success and efficiency of the examination procedure. Key Points WHO-recommended TE is unreliable or fails due to intercostal space-related factors. The 8th intercostal space on the mid-axillary line and end-inspiration are recommended. This standardized protocol aids in handling challenging cases and simplifies operational procedures. Graphical Abstract

Statistical analyses.Operator repeatability was assessed for intercostal width, stiffness, and SCD measurements using intra-class correlation coefficients (ICCs).The repeatability was classified based on ICC values as follows: poor (ICC:0-0.5),fair/moderate (ICC:0.5-0.75),good (ICC:0.75-0.9),and excellent (ICC:0.81-1.00)(1).To evaluate the demographic and anthropometric factors that possibly influenced the width and stiffness of the ICS, we performed univariate analysis using Spearman's correlation test for continuous candidate variables, and the Mann-Whitney U test for categorical candidate variables.Variables that achieved statistical significance (p <0.05) in univariate analysis were included in a multivariate model.The multiple regression model was constructed with age, height, weight, BMI, SCD, waist circumference, sex, and metabolic syndrome as candidate covariates, and the width and stiffness as the outcome variables.

Reliability of US measurement.
Of the 59 subjects, 34 participated in the intra-operator reliability analysis, while 24 were assessed for the inter-operator reliability (Supplementary Table 1).For the intra-operator reliability of width and SCD measurements, the ICCs(3,3) of end-inspiratory and end-expiratory conditions ranged between 0.972 and 0.992.Additionally, all ICCs(2,3) for inter-operator reliability were greater than 0.97.Both measures reflected excellent observer reliability.In comparison, inter-and intra-observer reliability for intercostal stiffness measurements were fair to good, with higher intra-operator ICC (3,3) (between 0.790 and 0.896) than inter-operator ICC(2,3) (between 0.593 and 0.683).This may imply the impact of operator experience, as the operator gained experience across two successive scans in an intra-operator setting.The lowest ICC(2,3) of 0.593 (95% CI: 0.060-0.824)was noted in the inter-observer reliability at end-inspiration.

Influencing factors on intercostal width and stiffness. Supplementary Table 2 and
Supplementary Table 3 show the results of both univariate and multivariate analyses evaluating the factors associated with the 16 experimental conditions.In univariate analysis (Supplementary Table 2), greater weight, higher BMI, and larger waist circumference were strongly associated with larger width values (p <0.001 in all conditions).Height was positively correlated to the width of the ICS at Sites 1 to 3, but not at Site 4 (end-inspiratory condition: p =0.140, end-expiration condition: p =0.201).An increase in SCD was fairly Insights Imaging (2024) Huang Z, Lam SK, Cheng LK, Lin Y, Zheng Y. associated with an increase in the width of the ICS at all sites (rs = 0.27-0.35).Wider ICSs were more likely to be identified in patients with metabolic syndrome (Site 1, end-expiratory condition: p = 0.012; Site 2, end-inspiratory condition: p = 0.002; Site 3, end-expiratory condition: p = 0.027; Site 4, end-inspiratory and end-expiratory conditions: p <0.001 and p = 0.003).There was a significant influence of gender on width values, with males having wider ICSs except for Site 3 (end-inspiratory condition: p = 0.138, end-expiratory condition: p = 0.124).Whereas no association between age and width was seen, older age fairly correlated with greater stiffness at one site (Site 3: rs = 0.37, p = 0.004).With the exception of height (Site 3: rs = -0.30,p = 0.021), none were significant predictors of the stiffness of the ICS.
In multivariate analysis (Supplementary Table 3), height, weight, BMI, SCD, gender, and the presence of metabolic syndrome remained independent factors associated with intercostal width.Waist circumference no longer provided a significant contribution to the width values in this model.The beta weights showed that weight (Site 1, end-inspiratory condition:  = -2.91,p = 0.041) had the greatest influence on the regression model, followed by BMI (Site 1, end-inspiratory condition:  = 2.75, p = 0.011; end-expiratory condition:  = 2.47, p = 0.018).However, only SCD and BMI were independently associated with the width values at most sites and appeared to be the most general determinants.The only covariate influencing the stiffness values was height ( = -2.03;p = 0.029), which was in line with the result of univariate analysis.

Supplementary Discussion
Measurement reliability.Excellent intra-and inter-operator agreement of the morphological measurements of width and SCD were observed, with all ICCs exceeding 0.97.In contrast, the measures of intercostal stiffness were less reliable, with ICCs ranging between 0.593 and 0.896.This reduction in ICCs may be attributed to SWE signal interference from adjacent ribs.A previous study (2) also reported the relatively low repeatability for SWE of the diaphragm (ICC(3,1) = 0.68) and intercostal muscles (ICC(3,1) = 0.44).Inter-operator ICCs(2,3) of intercostal SWE was found to be lower than intra-operator ICCs(3,3).We postulated that this variation may be due to discrepancies in the actual measurement site or differences in probe pressures applied between operators.It is worth noting that we attempted to minimize this measurement error by marking the investigated sites with a surgical skin marker to achieve higher repeatability.Placing the probe with minimal pressure was also required to prevent deformation of the intercostal tissues being examined.Despite these efforts, there is still room for improvement in terms of standardizing probe positioning to ensure reliable intercostal US measurements.
Influencing factors and implication for patient selection.Height was the only predictor of intercostal stiffness.Intercostal width, on the other hand, varied considerably depending on gender, BMI, and SCD.Only SCD had a homogeneous influence on the width across all sites, suggesting that subcutaneous fat accumulation contributes to a wider ICS.
We also confirmed narrower ICSs in women which aligns with a previous study (3).Our findings have clinical implications for patient selection and stratification based on patient characteristics: taller males with greater SCD and BMI tend to favour TE probe placement.
However, a dilemma arises, as the risk of TE failure might increase in this population due to the associated overmuch fat layers.Consequently, hepatologists are strongly urged to exercise caution when applying these regression results in pre-TE planning.

Table 2 .
Factors associated with the width and stiffness of the intercostal space in univariate analyses (n=59).

Table 3 .
Factors associated with the width and stiffness of the intercostal space in multivariate analyses.: age, height, weight, BMI, SCD, waist circumference, sex and metabolic syndrome were analysed by multiple linear regression.* p ≤ 0.05; BMI = body mass index; SCD = skin-liver capsule distance; ICS = intercostal space; β = standardized regression coefficient. Note